Somali pirates have stepped up their attacks and ignored monsoon storms to range the entire width of the Indian Ocean using hijacked gas and chemical tankers as "mother ships."

The dramatic expansion of their operations comes in the face of a campaign against them by European Union and NATO navies and is now costing the world's economy $12 billion a year.

Late last year the MV York, which was heading to the Seychelles, was hijacked, and now it is being used as a base for pirates who can use it to stay at sea for months and carry their lightweight attack craft.

Experts warn that this is just one of five similar vessels - some of which, like the MV York, are carrying cargos of gas or oil which make them almost impossible for foreign forces to attack them.

So far this year at least 20 ships have been attacked and about six captured - their crews being held in appalling conditions off the coastal towns of Hobyo and Haradheere.

Read more on this story about Somali pirates from Sky News.